Use GTK Pixbuf as fallback for downsampling images, if PIL is not available...some minor bugfixes in upload component detection...

New version using GTK eventbox instead of xhtml (looks cleaner and also provides the url in copy/paste operations)

Changed namespace to official namespace assigned by XEP-0363 (http://xmpp.org/extensions/xep-0363.html)

 * Content-type and content-length are checked for every URL in MUCs and regular chats
 * Plugin config now have a setting for max image download size (which is checked to content-length)
 * If content-type match and content-length is smaller then setting the image will be downloaded and displayed
 * If image can be loaded, the URL is no longer displayed (except as a tooltip on the image)
 * Image is clickable (opens browser with URL)
 * Images are displayed using XHTML
 * URL is displayed until image is displayed
Fixes #102, #105